Magnetic tape, which is a general-purpose magnetic recording medium, is typically produced by dispersing acicular ferromagnetic particles having a length in the long axis of 1 .mu.m or less in a binder solution along with appropriate additives (e.g., a dispersant, lubricant, and antistatic agent) to prepare a magnetic coating composition and then applying this coating composition on a poly(ethylene terephthalate) film (nonmagnetic support).
Binders for magnetic recording media are required, for example, to disperse ferromagnetic particles and hold them at high packing densities, to have no adverse effect on the orientation of the ferromagnetic particles, to give magnetic layers having good durability, abrasion resistance, heat resistance, and smoothness, and to have good adhesion to the nonmagnetic supports. Thus, binders are highly important. Conventionally employed binder resins include vinyl chloride-vinyl acetate copolymers, vinyl chloride-vinyl acetate-vinyl alcohol copolymers, vinyl chloride-vinylidene chloride copolymers, polyurethane resins, polyester resins, acrylonitrile-butadiene copolymers, nitrocellulose, cellulose acetate butyrate, epoxy resins, and acrylic resins.
Of those resins, polyurethane resins are superior in toughness, abrasion resistance, and other properties due to the intermolecular hydrogen bonding in the urethane bonds, but are not always satisfactory.
Since adipate-based or caprolactone-based conventional polyurethane resins are poor in such properties as anti-blocking properties, heat resistance, and running stability, such polyurethane resins are used particularly in combination with nitrocellulose or vinyl chloride-vinyl acetate copolymers. However, these blends are still insufficient in durability, abrasion resistance, and running stability, in the fields of video tapes, computer tapes, and floppy disks, where high performance is required.
For the purpose of improving the durability, abrasion resistance, and other properties of magnetic recording media, polyisocyanates are used as hardeners and various kinds of polyurethane resins having improved hardening reactivity with polyisocyanates are developed.
For example, polyurethanes in which hydroxyl groups are incorporated in the side chains thereof by special methods or by use of special raw materials are known. The incorporation of hydroxyl groups is accomplished by, for example, a method in which an isocyanate-terminated prepolymer is polymerized by use of a low molecular weight compound having three or more functional groups, such as glycerol, trimethylolpropane, or pentaerythritol, as part or all of the chain extender(s) or a method in which epoxy groups are first incorporated into a polyurethane resin and the epoxy groups then undergo ring opening to yield hydroxyl groups.
These polyurethane resins, however, are disadvantageous in that gelation of the polyurethane resins may occur during the production thereof and that the production process necessitates additional steps. The most serious problem encountered when such a polyurethane resin is used as a binder for ferromagnetic particles for magnetic recording media is that the more the hardening reactivity of the polyurethane resin with polyisocyanates is improved, the less the conventional polyurethane resins' ability to disperse ferromagnetic particles.
In order for magnetic recording media to show improved S/N ratios (signal-to-noise ratios) and to be recorded with information at high densities, the magnetic layers contain ferromagnetic particles which have smaller particle sizes and have been incorporated at high packing densities and oriented to high degrees, or the magnetic layers employ ferromagnetic particles that are more difficult to disperse, such as ferromagnetic metal particles.
The conventional polyurethane resins fail to meet the requirements for producing such high-performance magnetic recording media.
As described above, the conventional polyurethane resins cannot satisfy the requirements for the production of magnetic recording media which show improved S/N ratios and can be recorded with information at high recording densities, because the more the hardening reactivity of the polyurethane resins with polyisocyanates used as hardener is increased, to improve abrasion resistance or durability of magnetic recording media, the poorer is their ability to disperse magnetic particles.
It has thus been proposed to use, as a binder ingredient for magnetic layers, a polyurethane resin which has high reactivity with hardeners and good ability to disperse ferromagnetic particles. JP-A-62-226418 discloses a magnetic recording medium employing a polyurethane resin obtained by the reaction of an isocyanate-terminated prepolymer with a branched polyester polyol having a molecular weight of 500 to 5,000 in which a polycarboxylic acid or polyol having 3 or more carboxyl or hydroxyl groups has been copolymerized in an amount of 1 to 20 mol% based on the total amount of all the acid or alcohol ingredients copolymerized. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".) However, since the isocyanate-terminated prepolymer and the polyester polyol both have high molecular weights and high glass transition temperatures (Tg), the reactivity of the above-proposed polyurethane resin with hardeners remains insufficient.